gst: Use memcpy() instead of strncpy() where appropriate

strncpy() is assumed to be for strings so the compiler assumes that
it will need an extra byte for the string-terminaning NULL.

For cases where we know it's actually "binary" data, just copy it
with memcpy.

Fixes compiler warnings with gcc 8.

https://bugzilla.gnome.org/show_bug.cgi?id=795756
2 files changed